The production of oxirane compounds, such as propylene oxide and its higher homologs, is described in Kollar U.S. Pat. No. 3,351,635. In accordance with the Kollar process, the oxirane compound may be prepared by epoxidation of an olefinically unsaturated compound (for example, propylene) by use of organic hydroperoxide and a suitable metal catalyst, such as a molybdenum compound. Kollar teaches that activity of the metal catalyst disclosed therein for expoxidation of primary olefins is high and can lead to high selectivity of propylene to propylene oxide. These selectivities are obtained at high conversions of hydroperoxide (50% or higher) which conversion levels are important for commercial utilization of this technology. In accordance with the Kollar process, the epoxidation reaction proceeds under pressure in a liquid state, and accordingly, a liquid solution of the metal catalyst is desired.
In the preparation of metal compounds, for example, molybdenum salts, for the aforementioned purposes, various techniques have been used, many of which have been found to be extremely difficult to carry out efficiently on a commerical scale, and hence expensive, particularly for preparing hydrocarbon soluble compositions containing a high molybdenum content. Kollar U.S. Pat. No. 3,362,972 is concerned with preparation of molybdenum salts of carboxylic acids wherein molybdenum trioxide is reacted with oxalic acid in the presence of hexanoic acid. Sheng et al. U.S. Pat. No. 3,434,975 reports the preparation of molybdenum containing catalysts by reaction of molybdenum metal with peroxy compounds in the presence of a saturated alcohol. Sheng et al. U.S. Pat. No. 3,453,218 discloses the preparation of molybdenum containing epoxidation catalysts by reaction of molybdenum metal with a combination of tertiary butyl hydrogen peroxide and formic acid at low temperature. Ziolkowski et al. Polish Pat. No. 100,561 discloses the preparation of molybdenum-containing catalysts by treating Mo(OH).sub.5 with certain aliphatic or dicarboxylic acids and with an alpha-hydroxy acid or with certain diols or beta-diketones in an organic solvent. Ziolkowlski et al. Polish Pat. No. 103,742 is concerned with preparation of complexes of molybdenum from Mo(O)(OH).sub.3 and oxalic acid, lactic acid and ethylene glycol deposited on certain carriers. Sobczak et al., Journal Less-Common Met., Vol. 54, pp. 149-52 (1977) describe the reaction of molybdenum complexes with dicarboxylic acids, such as oxalic acid. Finally, Kuzimina et al., Izv. Trimiryazev. Sel'skokhoz Akad. (2 ), 224-8 (1970) describe complex forming reactions of molybdenyl ions with certain organic dicarboxylic acids, including oxalic acid.
However, each of these prior art processes are deficient by requiring expensive starting materials or forming carboxylates or complex molybdenum compositions which contain relatively low metal content and/or in requiring a number of steps in order to produce the desired high molybdenum-containing soluble catalyst composition.
Another molybdenum epoxidation catalyst is described by Bonetti et al in U.S. Pat. No. 3,480,563. Bonetti teaches that molybdenum trioxide may be reacted with a primary saturated acyclic alcohol having 4 to 22 carbon atoms or with a mono- or polyalkylene glycol monoalkyl ether. The reaction involves heating the molybdenum trioxide in the alcohol or ether to produce an organic soluble molybdenum catalyst.
Maurin et al in U.S. Pat. No. 3,822,321 describe oxidizing olefins with a hydroperoxide using a molybdenum catalyst prepared by reacting a molybdenum compound such as molybdic acid or a molybdic salt with a polyalcohol.
A molybdenum catalyzed epoxidation of olefins is described by Lines et al in U.S. Pat. No. 4,157,346. The catalyst is prepared by reacting an oxygen containing molybdenum compound with an amine (or an amine N-oxide) and alkylene glycol.
British Pat. No. 1,060,122 is concerned with an epoxidation reaction employing catalytic quantities of a molybdenum catalyst which is in the form of an inorganic molybdenum compound.
French Pat. No. 1,550,166 discloses that molybdic acid esters, and especially the glycol esters of molybdic acid, provide certain advantages over previously known catalysts to effect epoxidation employing organic hydroperoxides for reaction with olefinic compounds.
In U.S. Pat. No. 3,887,361 Lemke discloses that spent catalyst solutions obtained from the process of epoxidation of olefins with hydroperoxides in the presence of molybdenum may be treated to precipitate and separate dissolved molybdenum. The Lemke process involves mixing spent catalyst solution with 5 to 50 parts by weight of tertiary-butyl alcohol and heating the mixture to between 100.degree. C. and 300.degree. C. in a closed vessel or under reflux, thereby resulting in precipitation of molybdenum as a finely divided solid. The solid is disclosed to be suitable for recycle into further epoxidation reactions, as such, or optionally, after dissolution in an organic acid or an acid obtained in the "Oxo process" for production of oxygenated organic derivatives. The Lemke solids typically contain about 30 to about 40 percent by weight of molybdenum.
Accordingly, it is an object of the present invention to provide a simple, inexpensive method for the regeneration of molybdenum compositions from spent molybdenum-containing solid catalysts which compositions are characterized by a high metal content and are suitable for re-use in the epoxidation of olefins with organic hydroperoxides to produce the corresponding oxirane compounds.
A further object of the present invention is to provide a process for the preparation and/or reuse of molybdenum-containing epoxidation catalysts from spent molybdenum catalyst compositions which process is characterized by reduced hydroperoxide consumption and molybdenum losses and improved catalyst preparation productivity.